This invention relates to a method of modifying a pigment which is capable of obtaining a fine particle size as well as removing impurities from a crude pigment, and relates to an electrophotographic photoreceptor containing the modified pigment as the organic photoconductor.
Pigments are widely used in paints and for various other uses. The pigments used are not crude pigments as is but are provided with various modifications such as being homogeneously tinted, and having improved gloss, color strength, dispersibility or the like.
Heretofore, the modification of a crude pigment into a dispersible state is conducted by improving the synthesis conditions of the pigment, by a specific treatment after synthesis or the like. As the treatment after synthesis, there is the method of mixing and grinding the crude pigment with an inorganic salt or a grinding assistant, the method of blending a surfactant therewith and subsequent pulverization and so on. Furthermore, in the case of phthalocyanine compounds, there are the acid pasting method, the acid slurry method and so on. For the purification of pigment, the solvent extraction method is, in general, utilized.
Recently, various pigments have been used for photoelectric devices, electrophotographic photoreceptors, photorecording media and the like by utilizing the absorption wavelength of the pigment. The above functional pigments have problems in that their properties change during storage because of impurities and the properties become disproportionate because of uneven particle size, and the like. For example, when a crude pigment is used as the organic photoconductor of an electrophotographic photoreceptor, problems of the inferiority in uniform dispersability and of uneven sensitivity due to the larger particle size of the pigment occur. Moreover, when the pigment contains a large quantity of impurities, there are also problems in initial characteristics, such as a low charge acceptance and a great dark decay, and during repeated use, such as a sensitivity shift, a decrease in charge acceptance or the like occurring. In order to resolve the above problems, the above modifications have been developed, but they are still insufficient.